1. Field of the Invention
The present invention relates to an electrophotographic image-forming apparatus using a two-component developer and a print density control method thereof. More particularly, the present invention relates to an electrophotographic image-forming apparatus using a two-component developer and print density control method capable of adjusting a developing bias voltage to control a print density.
2. Description of the Related Art
Electrophotographic developing systems are generally employed in image-forming apparatuses such as photocopiers, laser beam printers (LBPs), light-emitting diode (LED) printers, and plain paper facsimile machines.
The electrophotographic developing system operates to develop electrostatic latent images formed on a photosensitive medium into visible images using developers and transfers the visible images onto a printing medium for printing. Such developing systems are mainly classified into a one-component developing system using a toner only, and a two-component developer such as a mixture of a carrier and a toner.
In the developing system using the two-component developer, it is important to control the ratio of toner to carrier in order to obtain a high-quality images. In other words, it is important to control the concentration of a developer because the concentration of a developer is an important factor that determines the image quality.
When the electrostatic latent image on the photosensitive medium is developed, a developing bias voltage applied to a developing roller determines an amount of toner to be finally supplied to the photosensitive medium. The developing bias voltage is generally set to a voltage of, for example, −500V, to enable optimum images to be obtained. However if the developing bias voltage varies when a surface potential of the photosensitive medium is maintained at a certain voltage of, for example, −700V due to a charging voltage, a potential difference between the surface potential of the photosensitive medium and the developing bias voltage increases or decreases so that too much or too little toner is supplied on the photosensitive medium, which leads to a higher or a lower print density to cause a secondary factor that degrades the print quality.
Accordingly, a method is needed which can control the developing bias voltage to not only solve the problem of degrading print quality but also to efficiently control the print density.